A conventional flat antenna remarkably deteriorates in its antenna characteristics when installed in or near a structure made of a material such as a high dielectric constant material or a metallic material, and accordingly becomes unable to function as an antenna. In view of this, a thin flat antenna has been practically used whose antenna characteristics are not affected by (i) the structure in which the antenna is installed or (ii) the structure located near the antenna.
Such an antenna has an antenna element provided on a dielectric substrate obtained by providing a dielectric member on a ground plane, and is called a patch antenna. The patch antenna typically has a shape of a square, a rectangle, or a circle. The patch antenna having either the square shape or the rectangular shape has a pair of sides each of which has a length given by the following formula:d=λ/2√∈eff or d=λ/4√∈eff 
where λ is the wavelength of the operating frequency, and ∈eff is the apparent dielectric constant of the dielectric substrate, and 1/√∈eff is the wavelength shortening rate.
The half-wavelength patch antenna has one side whose length corresponds to one-half of the wavelength, and has a feeding point positioned at any point on the side, excepting the midpoint of the side. On the other hand, a quarter-wavelength patch antenna has one side which has a length corresponding to one-quarter of the wavelength and which has one end electrically connected to a ground plane. The quarter-wavelength patch antenna has a feeding point placed at any point on the side.
The input impedance of the patch antenna depends on the position of the feeding point. Therefore, the feeding point is positioned such that the desired input impedance can be obtained. See a case of the patch antenna (circular patch antenna) having the circular shape. The circular patch antenna takes the shape of a circle whose circumferential length 2 πa is given by the following formula:2 πa=1.84λ/√∈eff 
where a is the radius of the circle.
The circular patch antenna is arranged such that the center of the circle is electrically connected to the ground plane, and such that the feeding is made with respect to any point except the center. The input impedance of the circular patch antenna also depends on the position of the feeding point. Therefore, the feeding point of the circular patch antenna is positioned such that the desired input impedance can be obtained.
As described above, the shape and size of the patch antenna are determined in accordance with the operating frequency and the effective dielectric constant of the dielectric substrate. In the meanwhile, the bandwidth of the patch antenna, i.e., an important antenna characteristic of the patch antenna, is determined in accordance with the thickness and a dielectric constant of the dielectric substrate. Specifically, as the dielectric substrate is thinner and has a larger dielectric constant, the bandwidth becomes narrower. In a general case, the patch antenna has a narrow bandwidth corresponding to not more than a bandwidth of 1% to 2% with respect to the operating frequency.
An antenna device using such a patch antenna element is disclosed, for example, in Japanese Unexamined Patent Publication No. 321718/1996 (Tokukaihei 8-321718; published on Dec. 3, 1996).
The antenna device disclosed in the publication is a patch antenna arranged in the following manner. That is, a pair of antenna elements are provided on a dielectric substrate having a rear surface on which a rectangular ground plane is formed, and respectively have sides electrically connected to the ground plane. Such an antenna device adopts a structure that improves (i) the balance between the powers supplied to the two antenna elements and (ii) a frequency characteristic corresponding to change in the phase difference between the powers.
However, the size and bandwidth of the antenna device such as the flat antenna having the antenna elements provided on the dielectric substrate provided on the ground plane depends on the dielectric constant of the dielectric substrate and on the operating frequency. This greatly limits freedom in setting the size and bandwidth of the antenna device. For example, in some cases, the patch antenna is too big to be installed in an electronic device having certain size and structure.
In contrast, see an inverted F antenna. As is the case with the patch antenna, the inverted F antenna can be installed on a surface of a metal case (metal structure) of an electronic apparatus or the like. However, unlike the patch antenna, the inverted F antenna is small, and can secure a wide band. However, it is structurally impossible for the inverted F antenna to be lower (thinner) in height (thickness). Therefore, the installation of the inverted F antenna on the surface of the metal structure causes such a problem that the surface of the metal structure is disfigured.
The patch antenna can be formed so as to be thinner than the inverted F antenna. However, the antenna element needs to have a side whose length corresponds to the length obtained by multiplying one-quarter of the wavelength by the wavelength shortening rate. Accordingly, the patch antenna requires an area more than five times as large as the inverted F antenna does. For example, a patch antenna using a dielectric substrate made of a glass plate having a relative dielectric constant of 6.91 and a thickness of 1.8 mm cannot cover the frequency band defined by wireless LAN (IEEE802.11b/g) 2.45 GHz. A required bandwidth for an antenna compliant with this wireless standard is at least 100 MHz.
Further, in the antenna device described in the above publication, the two antenna elements have the same frequency characteristic so as to attain a wide band, but cannot attain a wide band sufficient for a large number of channels used in the wireless LAN or the like.